Residual Stress Effects and Fatigue Behavior of Friction-Stir-Welded 2198-T8 Al-Li Alloy Joints

Abstract

Applications of friction stir welding in a fuselage structure were studied. Samples with two different friction-stirweld orientations in the fuselage panel were tested: one is along the fuselage longitudinal direction and the other one is along the fuselage circumferential direction. Then fatigue cracks were investigated that were set in three different types: parallel and perpendicular to friction stir welds and between double welds. Sample geometries were machined from identical welds in order to remove the effect of the weld process on fatigue behavior. Tests were conducted on M (T) specimens with either longitudinal or transverse welds. Cracks growing into or growing away from the weld center, as well as in the nugget zone, were investigated. It is shown that fatigue crack growth for cracks growing away from the center seems similar to that of the parent material; for crack growing in the nugget, crack grows slower than in the parent material; and for cracks starting between double welds, the crack grows slower than in the other two types. The virtual crack-closure technique method was used to calculate stress intensity factor from residual stress (K res ) and effective R ratio in an attempt to explain the experimental findings.Nomenclature a = crack length da=dN = crack growth rate E = Young's modulus of elasticity F j = reaction force on the jth node G = strain-energy release rate K = stress intensity factor K res = stress intensity factor from residual stress N = cycle (fatigue load) R eff = R ratio with the presence of residual stress R nom = applied load R ratio t = thickness of samples u i = total displacement from the ith node c = element size

Microstructure mapping of a friction stir welded AA2050 Al–Li–Cu in the T8 state

“…In the T8 (near peak-aged condition), alloys such as AA2198 containing about 3 wt%Cu and 1 wt%Li contain a high density of very thin T 1 plates of high aspect ratio (1.3 nm thickness vs. approximately 50 nm diameter) [5,9,10]. Although several authors have studied the potentiality of the latest generation of Al-Li-Cu for friction stir welds [11][12][13][14][15][16][17][18], only a limited number of studies are available which characterize their microstructure.…”

Analysis of fatigue crack growth behavior of SS 316(N) welds using the unified approach

“…Figure A shows the crack growth rates in the base and weld metals of 2195‐T8 Al‐Li alloys by Ma et al The authors have made FSW welds using different sizes of the specimen blanks keeping the thickness of the plate constant (8 mm). The figure shows that the crack growth rates differ with the size of the welds, indicating that the residual stresses can vary with the size of the welds.…”

Friction stir lap joining of 2198 aluminum–lithium alloy with weaving and pulsing variants

“…These alloys have also paired well with FSW as a joining method to offer an intriguing combination for manufacturers to consider [17,18]. The research community has taken notice as well, with a perhaps telling amount of work being conducted on FSW of Al-Li alloys [19][20][21][22][23][24][25][26][27][28][29][30][31], with particular emphasis on 2198. Other Al-Li alloys receiving attention including 2199, viewed as a viable candidate for aircraft skin, and 2099 extrusions, which would be used to form internal structural members [32].…”